Biaxially stretched polypropylene films have been suitably used as dielectric films for capacitors, such as high voltage capacitors, filter capacitors for various switching regulators, converters, inverters, and the like, and smoothing capacitors, among electric and electronic devices by utilizing excellent electrical properties, such as voltage resistance and low dielectric loss characteristics, and high moisture resistance of the polypropylene films. Polypropylene films have started to be used also as capacitors for inverter power supplies that regulate drive motors of electric cars, hybrid cars, and the like which have been increasingly in demand in recent years.
For such capacitors for inverter power supplies used in cars or the like, further reductions in size and weight of capacitors have been demanded along with the reductions in size and weight of vehicles. To reduce size and weight of a capacitor, as a film for capacitors, a polypropylene film with high stretching performance may be used and formed into an ultrathin film having a thickness of 1 to 6 μm, for example. Meanwhile, even for ultrathin films, high voltage resistance is required so that dielectric breakdown does not occur even when higher voltage is applied at higher temperatures. Furthermore, for capacitors based on such a film, it is required to have high voltage resistance, especially high initial voltage resistance and long-term voltage resistance, even when a high voltage is applied at a wide range of temperatures of −40° C. to 90° C., especially at high temperatures.
Regarding polypropylene film, for example, Patent Document 1 proposes to obtain a polypropylene film by biaxially-stretching a polypropylene resin having a predetermined number average molecular weight (Mn), molecular weight distribution (Mw/Mn or Mz/Mn), and melt flow rate to obtain a capacitor film that has excellent electrical insulation properties, antiblocking properties, and mechanical characteristics. However, the film described in Patent Document 1 is not expected to be used as a dielectric film for capacitors, and no investigation was conducted for effect on the voltage resistance at high temperatures.
Patent Document 2 proposes a biaxially stretched polypropylene film formed from a resin mixture obtained by adding, in a homopolymer (A) of isotactic polypropylene having a particular melt flow index (MFR), 1 mass % or more and 30 mass % or less of a homopolymer (B) of isotactic polypropylene having the MFR that is 1 to 30 g/10 min more than the MFR of the resin (A), to obtain an extremely thin surface-roughened biaxially stretched polypropylene film having excellent winding machining suitability. However, the polypropylene film formed from a resin, in which a large portion of the composition is low molecular components, may not always satisfy the high level of demands from the market in recent years regarding further reduction in the size of the capacitor element and regarding high initial voltage resistance and long-term durability at high temperatures.